A Reddit post from last week has sparked a discussion regarding iPhone performance as a function of battery age. While we expect battery capacity to decrease as batteries age, we expect processor performance to stay the same. However, users with older iPhones with lower-than-expected Geekbench 4 scores have reported that replacing the battery increases their score (as well as the performance of the phone). What's going on here? How many phones are experiencing decreased Geekbench 4 score?

To answer these questions I've plotted the kernel density of Geekbench 4 single-core scores for the iPhone 6s and the iPhone 7 running different versions of iOS. Scores obtained in low-power mode are not included in the distribution.

Evidence seems to be mounting that Apple is decreasing the clock speed of iPhones with decreased battery capacity to maintain the advertised battery life.